1. Field of the Invention
One or more embodiments of the invention relate to a constant voltage power supply circuit, and in particular to a constant voltage power supply circuit having an over-current protection function.
2. Background Art
As means for over-current protection of a constant voltage power supply circuit forming a series regulator, there are known a current limiting circuit which prevents an output current from becoming a current value greater than or equal to a predetermined current value, and an over-current protection circuit which limits an output current when output is short-circuited.
For example, JP H8-115135 A discloses a direct current stabilizing power supply circuit in which an input voltage is input to a series circuit of an over-current detecting resistor and a series transistor, stabilized, and output as a stabilized voltage to a load, wherein an over-current is detected by the over-current detecting resistor, and there is employed a drooping characteristic in which, when the output current exceeds a limit, the output voltage is gradually reduced while the current is set at the limit current, by an operation of a controlling transistor. In the drooping characteristic, a voltage which is approximately equal to an input voltage is applied between a drain and a source of the series transistor during output short-circuiting, a loss calculated as a product of this voltage and the output current becomes high, and the load to the series transistor becomes high. Therefore, this reference discloses the use of a fold-back type drooping characteristic or a modified fold-back type drooping characteristic where the output current is reduced stepwise depending on the output voltage, in place of the drooping characteristic.
Although the over-current protection function can be integrated in the constant voltage power supply circuit, due to variation in the built-in elements in the IC, the current values for the limitation of the over-current may vary. In some usages of the constant voltage power supply circuit, it may be desired that the limit current value for the over-current be strictly controlled. On the other hand, there are other usages where the limit current value may not need to be controlled so strictly.
As described above, an allowance range of the limit current value for the over-current differs depending on the usage. In order to handle these respective cases, there must be provided constant voltage power supply circuits having different specifications for the over-current protection function depending on the usages.